Interpretive Summary: Dryland crop yields in the central Great Plains increase linearly with amount of water used. Consequently knowing how much available water is stored in the soil at planting should be helpful in estimating yield and guiding a decision about crop selection or fallowing. This study tested the concept of using information on the amount of available soil water at planting in conjunction with assumptions about the amount of precipitation that might be received during a growing season to make a decision regarding what crop to plant or to fallow. Four decision rules were tested based on varying levels of expected growing season precipitation. Yield and net returns from the four crop selection methods were compared with yield and net returns from four set rotations. Using the decision rules that assumed below average or average growing season precipitation increased water use efficiency and precipitation use efficiency compared with the set rotational systems, but net returns were not different. Cropping frequency can be effectively increased in dryland cropping systems by use of crop selection rules based on water use/yield production functions, measured available soil water, and expected precipitation.

Technical Abstract:
No-till dryland cropping systems in the central Great Plains have more water available for crop production than the traditional conventionally tilled winter wheat (Triticum aestium L.)-fallow systems because of greater precipitation storage efficiency. That additional water is used most efficiently when a crop is present to transpire the water, and crop yields respond positively to increases in available soil water. The objective of this study was to evaluate yield, water use efficiency, precipitation use efficiency (PUE), and net returns of cropping systems where crop choice was based on established crop responses to water use while incorporating a grass/broadleaf rotation. Available soil water at planting was measured at several decision points each year and combined with three levels of expected growing season precipitation (70, 100, 130% of average) to provide input data for water use/yield production functions for seven grain crops and three forage crops. The predicted yields from those production functions were compared against established yield thresholds for each crop, and crops were retained for further consideration if the threshold yield was exceeded. Crop choice was then narrowed by following a rule which rotated summer crops with winter crops and also rotating grasses with broadleaf crops. Yields, water use efficiency, PUE, value-basis precipitation use efficiency ($PUE), gross receipts, and net returns from the four opportunity cropping (OC) selection schemes were compared with the same quantities from four set rotations [wheat-fallow (conventional till), (WF (CT)); wheat-fallow (no-till), (WF (NT)); wheat-corn (Zea mays L.)-fallow (no-till), (WCF); wheat-millet (Panicum miliaceum L.) (no-till), (WM)]. Water use efficiency was greater for three of the OC selection schemes than for any of the four set rotations. Precipitation was used more efficiently by using two of the OC selection schemes than by using any of the four set rotations. Net returns were greatest for the OC cropping decision method that assumed average growing season precipitation and allowed selection from all possible crop choices. The net returns from this system were not different from net returns from WF (CT) and WF (NT). Cropping frequency can be effectively increased in dryland cropping systems by use of crop selection rules based on water use/yield production functions, measured available soil water, and expected precipitation.